The present invention relates generally to antennas and more particularly to a multi-band planar inverted F antenna.
Planar inverted F antennas (PIFAs) are used in wireless communications, e.g., cellular telephones, wireless personal digital assistants (PDAs), wireless local area networks (LANs)—Bluetooth, etc. The PIFA generally includes a planar radiating element having a first area, and a ground plane having a second area that is parallel to the radiating element first area. An electrically conductive first line is coupled to the radiating element at a first contact located at an edge on a side of the radiating element. The first line is also coupled to the ground plane. An electrically conductive second line is coupled to the radiating element along the same side as the first line, but at a different contact location on the edge than the first line. The first and second lines are adapted to couple to a desired impedance, e.g., 50 ohms, at frequencies of operation of the PIFA. In the PIFA, the first and second lines are perpendicular to the edge of the radiating element to which they are coupled, thereby forming an inverted F shape (thus the descriptive name of planar inverted F antenna).
The resonance frequency of the PIFA is determined generally by the area of the radiating element and to a lesser extent the distance between the radiating element and the ground plane (thickness of the PIFA assembly). The bandwidth of the PIFA is generally determined by thickness of the PIFA assembly and the electrical coupling between the radiating element and the ground plane. A significant problem in designing a practical PIFA application is the trade off between obtaining a desired operating bandwidth and reducing the PIFA volume (area×thickness). Furthermore, it is preferable to have a larger ground plane area (shield) because this helps in reducing radio frequency energy that may enter into a user's head (SAR value=specific absorption rate), e.g., from a mobile cellular telephone. However, the volume of the PIFA increases with a larger ground plane area unless the thickness (distance between the radiating element and ground plane areas) is reduced.
As the number of wireless communications applications increases and the physical size of wireless devices decreases, antennas for these applications and devices are needed. Prior known planar inverted F antennas have sacrificed bandwidth by requiring a reduction in the volume (thickness) of the PIFA for a given wireless application.
In addition different markets use different operating frequencies. For example, a new GSM band at 850 MHz was assigned recently in North America. Existing PIF antenna solutions from the European GSM 900 MHz band need to be adapted properly, i.e., the resonance frequency needs to be shifted from 900 MHz to 850 MHz band. It is thus desirable to be able to redesign a wireless communication product for different frequencies with a minimum of design changes.
However, in order to use the same sort of antenna at a lower resonance frequency the physical dimensions need to be changed. As an example, the dimensions of a PIFA designed for 900 MHz need to be scaled by multiplying it with a factor 850/900 to operate at 850 Mhz. Therefore, it is obvious, that the dimensions of the PIF antenna are bigger at 850 MHz. Thus, redesigning a product for a different frequency can cause problems in the redesign of the respective antenna.
Therefore, there is a need for a PIFA design able to operate at a different resonance frequency without having to increase the dimensions thereof.